Pigment dispersions with polymeric dispersants having pending chromophore groups

ABSTRACT

A pigment dispersion includes a color pigment represented by formula (I): 
                         
wherein
 
R1 to R10 are independently selected from the group consisting of hydrogen, a halogen atom, a methyl group, an ethyl group, a methoxy group, an ethoxy group, —CF 3 , —COOH, —COOCH 3 , —SO 2 NH—C 6 H 5 , —CONH—C 6 H 5 , —CONH—C 6 H 5 —CONH 2 , and —CONH 2 ; and a polymeric dispersant, having via a linking group covalently linked to its polymeric backbone, at least one pending chromophore group which has a molecular weight smaller than 95% of the molecular weight of said color pigment. The pigment dispersion can be advantageously used in inkjet inks.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a 371 of PCT/EP2006/063490, filed Jun. 23, 2006.This application claims the benefit of U.S. Provisional Application No.60/713,012, filed Aug. 31, 2005, which is incorporated herein byreference in its entirety. In addition, this application claims thebenefit of European Application No. 05106456.6, filed Jul. 14, 2005,which is also incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to stable pigment dispersions andpigmented inkjet inks including color pigments that are stabilized bypolymeric dispersants having pending chromophore groups which exhibit astructural similarity with the color pigment.

2. Description of the Related Art

Pigment dispersions are made using a dispersant. A dispersant is asubstance for promoting the formation and stabilization of a dispersionof pigment particles in a dispersion medium. Dispersants are generallysurface-active materials having an anionic, cationic, or non-ionicstructure. The presence of a dispersant substantially reduces therequired dispersing energy. Dispersed pigment particles may have atendency to re-agglomerate after the dispersing operation due to mutualattraction forces. The use of dispersants also counteracts thisre-agglomeration tendency of the pigment particles.

The dispersant has to meet particularly high requirements when used forinkjet inks. Inadequate dispersing manifests itself as increasedviscosity in liquid systems, loss of brilliance, and/or hue shifts.Moreover, particularly good dispersion of the pigment particles isrequired to ensure unimpeded passage of the pigment particles throughthe nozzles of the print head, which are usually only a few micrometersin diameter. In addition, pigment particle agglomeration and theassociated blockage of the printer nozzles has to be avoided during thestandby periods of the printer.

Polymeric dispersants contain in one part of the molecule so-calledanchor groups, which adsorb onto the pigments to be dispersed. In aspatially separate part of the molecule, polymeric dispersants havepolymer chains sticking out whereby pigment particles are madecompatible with the dispersion medium, i.e., are stabilized.

The properties of polymeric dispersants depend on both the nature of themonomers and their distribution in the polymer. Polymeric dispersantsobtained by randomly polymerizing monomers (e.g., monomers A and Bpolymerized into ABBAABAB) or by polymerizing alternating monomers(e.g., monomers A and B polymerized into ABABABAB) generally result in apoor dispersion stability. Improvements in dispersion stability havebeen obtained using graft copolymer and block copolymer dispersants.

Graft copolymer dispersants consist of a polymeric backbone with sidechains attached to the backbone.

CA 2157361 (DU PONT) discloses pigment dispersions made by using a graftcopolymer dispersant with a hydrophobic polymeric backbone andhydrophilic side chains.

Block copolymer dispersants containing hydrophobic and hydrophilicblocks have been disclosed in numerous inkjet ink patents.

U.S. Pat. No. 5,859,113 (DU PONT) discloses an AB block copolymerdispersant with a polymeric A segment of polymerized glycidyl(meth)acrylate monomers reacted with an aromatic or aliphatic carboxylicacid, and a polymeric B segment of polymerized alkyl (meth)acrylatemonomers having 1-12 carbon atoms in the alkyl group, hydroxy alkyl(meth)acrylate monomers having about 1-4 carbon atoms in the alkylgroup.

In the design of polymeric dispersants for aqueous inkjet inks, theabove mentioned anchor groups, which adsorb onto the pigments to bedispersed, are generally hydrophobic groups exhibiting an affinity forthe pigment surface.

EP 0763580 A (TOYO INK) discloses an aqueous type pigment dispersingagent having a portion which has a high affinity with a pigment andwhich has at least one type selected from the group consisting of anorganic dye, anthraquinone, and acridone only at a terminal end or atboth terminal ends of at least one aqueous polymer selected from thegroup consisting of an aqueous linear urethanic polymer and an aqueouslinear acrylic polymer. EP 0763378 A (TOYO INK) discloses similarpigment dispersing agents for non-aqueous pigment dispersions.

U.S. Pat. No. 5,420,187 (TOYO INK) discloses a pigment-dispersing agentobtained by polymerizing an addition-polymerizable monomer having anacidic functional group and another addition-polymerizable monomer inthe presence of a polymerization initiator, the polymerization initiatorbeing a diazotization product prepared by diazotizing at least onecompound selected from the group consisting of an anthraquinonederivative having an aromatic amino group, an acridone derivative havingan aromatic amino group, and an organic dyestuff having an aromaticamino group. In this pigment-dispersing agent, the colorant is locatedin the polymeric backbone itself.

US 2003/0044707 (TOYO INK) discloses a dispersing agent for a pigment,including a specific compound having a structure wherein aphthalocyanine type molecular skeleton which is adsorptive on thepigment and an oligomer unit or polymer unit which preventsre-agglomeration of the pigment to bring out the effect of dispersionare covalently bonded, and having affinity for a medium or a solvent.

Current practice is that the exact or almost the exact chemicalstructure of the color pigment is incorporated as the anchor group inthe polymeric dispersing agent to assure maximum affinity with the colorpigment. As a consequence, each pigment has its own tailor-madepolymeric dispersant. In practice, this requires the holding of aninventory of different polymeric dispersants for producing a completerange of color inkjet ink sets. The cyan ink with copper phthalocyanineas the pigment is a rare exception in that all desired properties arecombined in the same pigment. But yellow pigments have to be selectedbased on the properties that are the most important in their applicationof the inkjet ink. For example, some yellow pigments are selected fortheir light stability, while others are selected to obtain images havinghigh color strength. The holding of such an inventory of different typesof polymeric dispersants incurs financial penalties due to additionalstorage and logistical requirements as well as increasing thepossibility of using the “wrong” polymeric dispersant for the productionof a particular inkjet ink. Another disadvantage is that the lowsolubility of the pigment generally complicates the synthesis of suchpolymeric dispersants.

For consistent image quality, the inkjet ink requires a dispersionstability capable of dealing with high temperatures (above 60° C.)during transport of the ink to a customer and changes in the dispersionmedium of the inkjet ink during use, for example, evaporation of solventand increasing concentrations of humectants, penetrants, and otheradditives.

Therefore, it is highly desirable to be able to manufacture a range ofstable pigmented inkjet inks using a single polymeric dispersantobtained by simple synthesis.

SUMMARY OF THE INVENTION

In order to overcome the problems described above, preferred embodimentsof the present invention provide inkjet inks using a polymericdispersant obtained by uncomplicated synthesis and suitable fordifferent color pigments.

Further preferred embodiments of the present invention provide inkjetinks with high dispersion stability.

Further preferred embodiments of the present invention provide inkjetinks producing images of high image quality with a high optical density.

Further preferred embodiments of the present invention will becomeapparent from the description hereinafter.

It has been surprisingly discovered that inkjet inks with high opticaldensity and high stability are obtained using a colored polymericdispersant wherein a pending chromophore group exhibits a structuralsimilarity with the color pigment, but is smaller in size than the colorpigment.

A further preferred embodiment of the present invention has beenachieved with a pigment dispersion including a color pigment representedby formula (I):

wherein,R1 to R10 are independently selected from the group consisting ofhydrogen, a halogen atom, a methyl group, an ethyl group, a methoxygroup, an ethoxy group, —CF₃, —COOH, —COOCH₃, —SO₂NH—C₆H₅, —CONH—C₆H₅,—CONH—C₆H₅—CONH₂ and —CONH₂;and a polymeric dispersant having, via a linking group covalently linkedto its polymeric backbone, at least one pending chromophore group whichhas a molecular weight smaller than 95% of the molecular weight of thecolor pigment.

Other features, elements, processes, steps, characteristics andadvantages of the present invention will become more apparent from thefollowing detailed description of preferred embodiments of the presentinvention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Definitions

The term “colorant”, as used in the preferred embodiments of the presentinvention, means dyes and pigments.

The term “dye”, as used in the preferred embodiments of the presentinvention, means a colorant having a solubility of 10 mg/L or more inthe medium in which it is applied and under the ambient conditionspertaining thereto.

The term “pigment” is defined in DIN 55943, herein incorporated byreference, as a coloring agent that is practically insoluble in theapplication medium under the pertaining ambient conditions, hence havinga solubility of less than 10 mg/L therein.

The term “chromophore group”, as used in the preferred embodiments ofthe present invention, means a group with an absorption maximum between300 nm and 2,000 nm.

The term “pending chromophore group”, as used in the preferredembodiments of the present invention, means a chromophore groupoccurring as a side group on the polymeric backbone and not a group inthe polymeric backbone itself or occurring solely as an end group of thepolymeric backbone.

The term “C.I.” is used in the preferred embodiments of the presentapplication as an abbreviation for Color Index.

The term “actinic radiation” as used in the preferred embodiments of thepresent invention, means electromagnetic radiation capable of initiatingphotochemical reactions.

The term “DP” is used in the preferred embodiments of the presentapplication as an abbreviation for degree of polymerization, i.e., thenumber of structural units (monomers) in the average polymer molecule.

The term “PD” is used in the preferred embodiments of the presentapplication as an abbreviation for polydispersity of a polymer.

The term “dispersion”, as used in the preferred embodiments of thepresent invention, means an intimate mixture of at least two substances,one of which, called the dispersed phase or colloid, is uniformlydistributed in a finely divided state through the second substance,called the dispersion medium.

The term “polymeric dispersant”, as used in the preferred embodiments ofthe present invention, means a substance for promoting the formation andstabilization of a dispersion of one substance in a dispersion medium.

The term “copolymer”, as used in the preferred embodiments of thepresent invention means a macromolecule in which two or more differentspecies of monomer are incorporated into a polymer chain.

The term “block copolymer”, as used in the preferred embodiments of thepresent invention, means a copolymer in which the monomers occur inrelatively long alternate sequences in a chain.

The term “spectral separation factor” as used in the preferredembodiments of the present invention means the value obtained bycalculating the ratio of the maximum absorbance A_(max) (measured atwavelength λmax) over the reference absorbance A_(ref) determined at ahigher wavelength λref.

The abbreviation “SSF” is used in the preferred embodiments of thepresent invention for spectral separation factor.

The term “alkyl” means all variants possible for each number of carbonatoms in the alkyl group, i.e., for three carbon atoms: n-propyl andisopropyl; for four carbon atoms: n-butyl, isobutyl, and tertiary-butyl;for five carbon atoms: n-pentyl, 1,1-dimethyl-propyl,2,2-dimethylpropyl, and 2-methyl-butyl etc.

The term “acyl group” means —(C═O)-aryl and —(C═O)— alkyl groups.

The term “aliphatic group” means saturated straight chain, branchedchain, and alicyclic hydrocarbon groups.

The term “unsaturated aliphatic group” means straight chain, branchedchain, and alicyclic hydrocarbon groups which contain at least onedouble or triple bond.

The term “aromatic group” as used in the preferred embodiments of thepresent invention means an assemblage of cyclic conjugated carbon atoms,which are characterized by large resonance energies, e.g., benzene,naphthalene, and anthracene.

The term “alicyclic hydrocarbon group” means an assemblage of cycliccarbon atoms, which do not form an aromatic group, e.g., cyclohexane.

The term “heteroaromatic group” means an aromatic group wherein at leastone of the cyclic conjugated carbon atoms is replaced by a non-carbonatom such as a nitrogen atom, a sulphur atom, a phosphorous atom, aselenium atom, and a tellurium atom.

The term “heterocyclic group” means an alicyclic hydrocarbon groupwherein at least one of the cyclic carbon atoms is replaced by an oxygenatom, a nitrogen atom, a phosphorous atom, a silicon atom, a sulphuratom, a selenium atom, or a tellurium atom.

Pigmented Inkjet Ink

The pigmented inkjet ink according to a preferred embodiment of thepresent invention contains at least three components: (i) a colorpigment, (ii) a polymeric dispersant, and (iii) a dispersion medium.

The pigmented inkjet ink according to a preferred embodiment of thepresent invention may further contain at least one surfactant.

The pigmented inkjet ink according to a preferred embodiment of thepresent invention may further contain at least one biocide.

The pigmented inkjet ink according to a preferred embodiment of thepresent invention may further contain at least one humectant and/orpenetrant.

The pigmented inkjet ink according to a preferred embodiment of thepresent invention may further contain at least one pH adjuster.

The pigmented inkjet ink according to a preferred embodiment of thepresent invention may contain at least one humectant to prevent theclogging of the nozzle due to its ability to slow down the evaporationrate of ink.

The viscosity of the pigmented inkjet ink according to a preferredembodiment of the present invention is preferably lower than 100 mPa·s,more preferably lower than 30 mPa·s, and most preferably lower than 15mPa·s at a shear rate of 100 s⁻¹ and a temperature between 20° C. and110° C.

The pigmented inkjet ink according to a preferred embodiment of thepresent invention is preferably an aqueous solvent based or an oil basedpigmented inkjet ink.

The pigmented inkjet ink according to a preferred embodiment of thepresent invention may be curable and may contain monomers, oligomers,and/or prepolymers possessing different degrees of functionality. Amixture including combinations of mono-, di-, tri- and/or higherfunctionality monomers, oligomers, or prepolymers may be used. Theinitiator typically initiates the polymerization reaction. A catalystcalled an initiator for initiating the polymerization reaction may beincluded in the curable pigmented inkjet ink. The initiator can be athermal initiator, but is preferably a photo-initiator. Thephoto-initiator requires less energy to activate than the monomers,oligomers, and/or prepolymers to form the polymer. The photo-initiatorsuitable for use in the curable fluid may be a Norrish type I initiator,a Norrish type II initiator, or a photo-acid generator.

Color Pigments

The color pigment used in the pigmented inkjet ink according to apreferred embodiment of the present invention is represented by formula(I):

whereinR1 to R10 are independently selected from the group consisting ofhydrogen, a halogen atom, a methyl group, an ethyl group, a methoxygroup, an ethoxy group, —CF₃, —COOH, —COOCH₃, —SO₂NH—C₆H₅, —CONH—C₆H₅,—CONH—C₆H₅—CONH₂, and —CONH₂;and a polymeric dispersant having, via a linking group covalently linkedto its polymeric backbone, at least one pending chromophore group whichhas a molecular weight smaller than 95% of the molecular weight of thecolor pigment.

In a preferred embodiment, R2, R4, R5, R7, R9, and R10 in Formula (I)are hydrogen.

The color pigment may be chosen from those disclosed by HERBST et al.,Industrial Organic Pigments, Production, Properties, Applications; 3rdEdition, Wiley-VCH, 2004, ISBN 3527305769.

Particular preferred pigments are C.I. Pigment Yellow 155 and 198.

The pigment particles in the pigmented inkjet ink should be sufficientlysmall to permit free flow of the ink through the inkjet printing device,especially at the ejecting nozzles. It is also desirable to use smallparticles for maximum color strength and to slow down sedimentation.

The average particle size of the pigment in the pigmented inkjet inkshould be between 0.005 μm and 15 μm. Preferably, the average pigmentparticle size is between 0.005 μm and 5 μm, more preferably between0.005 μm and 1 μm, particularly preferably between 0.005 μm and 0.3 μmand most preferably between 0.040 μm and 0.150 μm. Larger pigmentparticle sizes may be used as long as the advantages of the presentinvention are achieved.

The pigment is preferably used in the pigmented inkjet ink in an amountof 0.1 wt % to 20 wt %, more preferably 1 wt % to 10 wt % based on thetotal weight of the pigmented inkjet ink.

Polymeric Dispersants

The polymeric dispersant used in the pigmented inkjet ink according to apreferred embodiment of the present invention contains one or morepending chromophore groups linked by a linking group to a polymericbackbone.

The polymeric dispersant used in the pigmented inkjet ink according to apreferred embodiment of the present invention preferably has a polymericbackbone with a polymerization degree DP between 5 and 1,000, morepreferably between 10 and 500, and most preferably between 10 and 100.

The polymeric dispersant used in the pigmented inkjet ink according to apreferred embodiment of the present invention preferably has a numberaverage molecular weight Mn between 500 and 30,000, more preferablybetween 1,500 and 10,000.

The polymeric dispersant preferably has a polymeric dispersity PDsmaller than 2, more preferably smaller than 1.75, and most preferablysmaller than 1.5.

The polymeric dispersant is preferably used in the pigmented inkjet inkin an amount of 5 wt % to 600 wt %, more preferably 10 wt % to 100 wt %based on the weight of the pigment.

Polymeric Backbones

The polymeric backbone of the polymeric dispersant used in the pigmentedinkjet ink according to preferred embodiments of the present inventionis required for the compatibility between polymeric dispersant anddispersion medium.

It is not required that the polymeric backbone has an affinity for thepigment. For example, the polymeric backbone of a dispersant for aqueousinkjet inks can be a homopolymer of acrylic acid monomers. A homopolymeris generally incapable of dispersing pigments, but the presence of apending chromophore group exhibiting a similarity with the pigmentensures an adequate affinity between polymeric dispersant and pigmentsurface.

The polymeric backbone can also be a statistical copolymer, a blockcopolymer, a graft copolymer, a comb polymer, or an alternatingcopolymer. Also suitable as the polymeric backbone is a gradientcopolymer as disclosed by MATYJASZEWSKI et al., Atom Transfer RadicalPolymerization, Chem. Reviews 2001, Vol. 101, pp. 2921-2990. Sometimesit can be useful to include a number of monomers with a high affinityfor the pigment surface to enhance certain properties of the inks, e.g.,dispersion stability. For example, the polymeric backbone of adispersant for aqueous inkjet inks may contain hydrophobic monomers toincrease the affinity of the polymeric dispersant for the pigmentsurface. However, in enhancing this affinity for the pigment surface,care should be taken that enough of the polymeric backbone sticks out tomake the pigment particles compatible with the dispersion medium.

In graft copolymers, the use of grafted chains ofmethoxypolyethyleneglycol (MPEG) has been found to be very advantageousin aqueous inkjet inks. For solvent-based inkjet inks the use of graftedchains of polyester were found to be very advantageous. A preferred MPEGmacromonomer is BISOMER™ MPEG 350MA (methoxypolyethyleneglycolmethacrylate) from LAPORTE INDUSTRIES LTD.

Preferred grafted chains of polyester in non-aqueous inkjet inks arederived from δ-valerolactone, ε-caprolactone, and/or C₁ to C₄ alkylsubstituted ε-caprolactone. The grafted chains can be introduced intothe polymeric dispersant through CDI coupling of a polyester-OH chainwith a carboxylic acid group of, for example, an acrylic acid monomer inthe polymeric backbone of the dispersant. However, it was observed thatgrafting by free radical polymerization, wherein the polyester chainalready coupled to the carboxylic acid group of an acrylic acid monomerwas used as a macro-monomer, not only resulted in better dispersionquality and stability of the inkjet inks but also obtained a morereproducible polymeric dispersant synthesis requiring less purification.

For radiation curable inks with the dispersion medium including orconsisting of monomers and/or oligomers, many (co)polymers having goodsolubility in the dispersion medium were found to be suitable for thepolymer backbone of the polymeric dispersant.

The copolymeric backbone preferably consists of no more than 2 or 3monomer species.

The monomers and/or oligomers used to prepare the polymeric dispersantused in the pigmented inkjet ink according to a preferred embodiment ofthe present invention can be any monomer and/or oligomer found in thePolymer Handbook, Vol. 1+2, 4th Edition, edited by J. BRANDRUP et al.,Wiley-Interscience, 1999.

Suitable examples of monomers include: acrylic acid, methacrylic acid,maleic acid, acryloyloxybenzoic acid and methacryloyloxybenzoic acid (ortheir salts), and maleic anhydride; alkyl(meth)acrylates (linear,branched, and cycloalkyl) such as methyl(meth)acrylate,n-butyl(meth)acrylate, tert-butyl(meth)acrylate,cyclohexyl(meth)acrylate, and 2-ethylhexyl(meth)acrylate;aryl(meth)acrylates such as benzyl(meth)acrylate andphenyl(meth)acrylate; hydroxyalkyl(meth)acrylates such ashydroxyethyl(meth)acrylate and hydroxypropyl(meth)acrylate;(meth)acrylates with other types of functionalities (e.g., oxirane,amino, fluoro, polyethylene oxide, phosphate-substituted) such asglycidyl (meth)acrylate, dimethylaminoethyl(meth)acrylate,trifluoroethyl acrylate, methoxypolyethyleneglycol (meth)acrylate, andtripropyleneglycol(meth)acrylate phosphate; allyl derivatives such asallyl glycidyl ether; styrenics such as styrene, 4-methylstyrene,4-hydroxystyrene, and 4-acetoxystyrene; (meth)acrylonitrile;(meth)acrylamides (including N-mono and N,N-disubstituted) such asN-benzyl (meth)acrylamide; maleimides such as N-phenyl maleimide,N-benzyl maleimide, and N-ethyl maleimide; vinyl derivatives such asvinylcaprolactam, vinylpyrrolidone, vinylimidazole, vinylnaphthalene,and vinyl halides; vinylethers such as vinylmethyl ether; andvinylesters of carboxylic acids such as vinylacetate and vinylbutyrate.

Linking Groups

The pending chromophore group is preferably linked by a linking group tothe polymeric backbone. The linking group preferably contains at leastone carbon atom, one nitrogen atom, one oxygen atom, one phosphorousatom, one silicon atom, one sulphur atom, one selenium atom, or onetellurium atom.

The linking group preferably has a molecular weight smaller than themolecular weight of the pending chromophore group, more preferably thelinking group has a molecular weight smaller than 80% of the molecularweight of the pending chromophore group, and most preferably the linkinggroup has a molecular weight smaller than 50% of the molecular weight ofthe pending chromophore group.

In a preferred embodiment, the linking group is the result ofmodification of a (co)polymer with a chromophore having a reactivegroup. Suitable reactive groups on the chromophore include thiol groups,primary or secondary amino groups, carboxylic acid groups or saltsthereof, hydroxyl groups, isocyanate groups, and epoxy groups. Typicalcovalent bonds formed by reaction of the chromophore with the polymericbackbone include an amide, an ester, a urethane, an ether, and athioether.

In another preferred embodiment, the polymeric dispersant is prepared bycopolymerizing monomers of the polymeric backbone and monomerscontaining a chromophore group. In this case, the linking group isalready present in the monomer. This polymerization method offers theadvantage of well-controlled design of polymeric dispersants for a widevariety of dispersion media. Due to its low solubility, a monomercontaining the complete color pigment as a chromophore group posesproblems both in the synthesis of the polymeric dispersants, as well asthe suitability of the polymeric dispersant for a wide variety ofdispersion media and pigments.

Pending Chromophore Groups

The pending chromophore group of the polymeric dispersant used in thepigmented inkjet ink according to a preferred embodiment of the presentinvention exhibits a high similarity with the color pigment of thepigmented inkjet ink, and has a molecular weight which is smaller than90%, preferably smaller than 85%, more preferably smaller than 75%, andmost preferably smaller than 65% of the molecular weight of the colorpigment.

The pending chromophore group of the polymeric dispersant may berepresented by formula (II):

whereinone of L1, L2, or L3 is the linking group and is selected from the groupconsisting of an aliphatic group, a substituted aliphatic group, anunsaturated aliphatic group, and a substituted unsaturated aliphaticgroup;L1, L2, and/or L3, if not representing the linking group, areindependently selected from the group consisting of hydrogen, an alkylgroup, an alkenyl group, an alkoxy group, a carboxylic acid group, anester group, an acyl group, a nitro group, and a halogen;AR1 and AR2 represent an aromatic group; andn represents the integer 0 or 1.

The linking groups L1 and L3 in the pending chromophore group accordingto Formula (II) consist of all atoms between the polymeric backbone andthe first atom of the aromatic group by which the pending chromophoregroup is linked to the polymeric backbone. The linking group L2 in thepending chromophore group according to Formula (II) consists of allatoms between the polymeric backbone and the carbon atom directly linkedto both L2 and the carbonyl group of the acetoacetanilide group in thepending chromophore group according to Formula (II).

In a preferred embodiment, the pending chromophore group of thepolymeric dispersant may be represented by formula (III):

whereinone of R1 to R11 is the linking group forming a covalent bond with thepolymeric backbone;R1 to R11, if not representing the linking group, are independentlyselected from the group consisting of hydrogen, an alkyl group, analkenyl group, an alkoxy group, an alcohol group, a carboxylic acidgroup, an ester group, an acyl group, a nitro group, and a halogen; orR7 and R8 may together form a heterocyclic ring. Preferably theheterocyclic ring formed by R7 and R8 is imidazolone or2,3-dihydroxypyrazine, so that a benzimidazolone ring and a2,3-dihydroxyquinoxaline ring respectively are formed in Formula (III).

Suitable examples of the pending chromophore group represented byformula (III) having an unreacted linking group include:

The pending chromophore group is preferably present in the range of 1 to30 percent, more preferably in the range 5 to 20 percent based on themonomeric units of the polymeric backbone. Polymeric dispersants havinga homopolymer or statistical copolymer as the polymeric backbone withmore than 45 percent of the monomeric units of the polymeric backbonehaving pending chromophore groups exhibit problems of solubility of thepolymeric dispersant in the dispersion medium and deterioration of thedispersing properties due to the fact that the dispersant would go flaton the pigment surface. However, in the case of a well-definedblock-copolymer, a good dispersion can be obtained with 50 percent ofthe monomeric units of the polymeric backbone having pending chromophoregroups. This well-defined block-copolymer preferably has at least oneblock containing no pending chromophore groups.

In some cases, the dispersion stability of the pigment according to apreferred embodiment of the present invention can be further improved byincreasing the number of pending chromophore groups in the polymericdispersant. In a preferred embodiment, two, three, or more pendingchromophore groups are located in close proximity of each other on thepolymeric backbone. Close proximity means preferably less than 50monomeric units, more preferably less than 20 monomeric units, and mostpreferably less than 10 monomeric units between two pending chromophoregroups. It is believed that the dispersion stability improvement by morepending chromophore groups is due to the dynamic character of theattaching and detaching of the pending chromophore group to the pigmentsurface. By increasing the number of pending chromophore groups, theprobability that all pending chromophore groups will be in a “detachedstate” at the same time is expected to decrease.

Synthesis

The polymerization process may be a condensation polymerization, inwhich the chain growth is accompanied by elimination of small moleculessuch as water or methanol or an addition polymerization, in which thepolymer is formed without the loss of other materials. Polymerization ofthe monomers can be conducted according to any conventional method suchas bulk polymerization and semi-continuous polymerization.

The synthesis is preferably performed by a controlled radicalpolymerization (CRP) technique. Suitable polymerization techniquesinclude ATRP (atom transfer radical polymerization), RAFT (reversibleaddition-fragmentation chain transfer polymerization), MADIX (reversibleaddition-fragmentation chain transfer process, using a transfer activexanthate), catalytic chain transfer (e.g., using cobalt complexes), GTP(group transfer polymerization), or nitroxide (e.g. TEMPO) mediatedpolymerizations.

In a preferred embodiment, the polymeric dispersant used in thepigmented inkjet ink is prepared by a post-polymerization modificationwith a chromophore. The chromophore is covalently linked to the polymerbackbone of the polymeric dispersant. The post-polymerizationmodification can be any suitable reaction, e.g., an esterificationreaction.

An esterification reaction suitable for post-polymerization modificationcan be performed using N,N′-carbonyldiimidazole (CDI). In a first step,the carboxylic moieties of the polymer are activated with CDI to form anintermediate imidazole, which is then esterified with the chromophorehaving a reactive hydroxyl group. The completion of the first step canbe observed when the CO₂ degassing stops. Synthesis scheme with Rrepresenting the chromophore group:

Side products of the reaction can be removed by acidifying the aqueousmedium used for precipitation of the polymer (hydrolysis of remainingactivated esters and protonation of imidazole that remains in water, inthis way achieving the separation from the polymer). If the finalmodified polymer is water-soluble (e.g., modified homopolymer of acrylicacid), a dialysis can be performed to purify the polymer.

In another preferred embodiment, the polymeric dispersant used in thepigmented inkjet ink is prepared by a copolymerization with a monomercontaining a chromophore group. It was observed that the pigment-basedmonomers containing a chromophore group were stable in the presence ofradicals. Classical free radical polymerization (FRP) techniques forpreparing statistical copolymers in a one reactor polymerization andATRP for preparing block copolymers were possible to prepare thepolymeric dispersant.

Monomers with a Chromophore Group

A monomer with a chromophore group for the preparation of the polymericdispersant used in the pigmented inkjet ink according to a preferredembodiment of the present invention can be represented by the generalformula:A-L-BwhereinA represents a polymerizable functional group, preferably anethylenically unsaturated polymerizable functional group;L represents a divalent linking group; andB represents a chromophore group.

In a preferred embodiment, the ethylenically unsaturated polymerizablegroup is selected from the group consisting of a styrene, an acrylate, amethacrylate, an acrylamide, a methacrylamide, a maleimide, a vinylester, and a vinyl ether.

The monomer with a chromophore group can be represented by Formula(GEN-I):

whereinAR₁ and AR₂ represent a substituted or unsubstituted aromatic group andR represents a substituted or unsubstituted aliphatic group, with theproviso that one of R, AR₁, and AR₂ has a substituent with apolymerizable functional group, preferably an ethylenically unsaturatedpolymerizable functional group.

In a preferred embodiment, AR₂ of Formula (GEN-I) is replaced by analkyl group, preferably methyl or ethyl.

In another preferred embodiment, AR₂ of Formula (GEN-I) is replaced byan aliphatic substituent with a polymerizable functional group,preferably an ethylenically unsaturated polymerizable functional group.Preferably this aliphatic ethylenically unsaturated polymerizablefunctional group is represented by

Suitable monomers according to Formula (GEN-I) include the monomersdisclosed in Table 1: styrene derivatives, in Table 2: (meth)acrylateand (meth)acrylamide derivatives, and in Table 3: other polymerizablederivatives.

TABLE 1

TABLE 2

TABLE 3

Dispersion Media

The dispersion medium used in the pigmented inkjet ink according to apreferred embodiment of the present invention is a liquid. Thedispersion medium may consist of water and/or organic solvent(s).

If the pigmented inkjet ink is a curable pigmented inkjet ink, waterand/or organic solvent(s) are replaced by one or more monomers and/oroligomers to obtain a liquid dispersion medium. Sometimes, it can beadvantageous to add a small amount of an organic solvent to improve thedissolution of the dispersant. The content of organic solvent should belower than 20 wt % based on the total weight of the pigmented inkjetink.

Suitable organic solvents include alcohols, aromatic hydrocarbons,ketones, esters, aliphatic hydrocarbons, higher fatty acids, carbitols,cellosolves, and higher fatty acid esters. Suitable alcohols include,methanol, ethanol, propanol and 1-butanol, 1-pentanol, 2-butanol, andt.-butanol. Suitable aromatic hydrocarbons include toluene and xylene.Suitable ketones include methyl ethyl ketone, methyl isobutyl ketone,2,4-pentanedione, and hexafluoroacetone. Also, glycol, glycolethers,N-methylpyrrolidone, N,N-dimethylacetamid, and N, N-dimethylformamid maybe used.

Suitable monomers and oligomers can be found in Polymer Handbook, Vol.1+2, 4th Edition, edited by J. BRANDRUP et al., Wiley-Interscience,1999.

Suitable examples of monomers for curable pigmented inkjet inks include:acrylic acid, methacrylic acid, maleic acid (or their salts), and maleicanhydride; alkyl(meth)acrylates (linear, branched and cycloalkyl) suchas methyl(meth)acrylate, n-butyl(meth)acrylate,tert-butyl(meth)acrylate, cyclohexyl(meth)acrylate, and2-ethylhexyl(meth)acrylate; aryl(meth)acrylates such asbenzyl(meth)acrylate and phenyl(meth)acrylate;hydroxyalkyl(meth)acrylates such as hydroxyethyl(meth)acrylate andhydroxypropyl(meth)acrylate; (meth)acrylates with other types offunctionalities (e.g., oxirane, amino, fluoro, polyethylene oxide,phosphate-substituted) such as glycidyl (meth)acrylate,dimethylaminoethyl(meth)acrylate, trifluoroethyl acrylate,methoxypolyethyleneglycol (meth)acrylate, andtripropyleneglycol(meth)acrylate phosphate; allyl derivatives such asallyl glycidyl ether; styrenics such as styrene, 4-methylstyrene,4-hydroxystyrene, and 4-acetoxystyrene; (meth)acrylonitrile;(meth)acrylamides (including N-mono and N,N-disubstituted) such asN-benzyl (meth)acrylamide; maleimides such as N-phenyl maleimide,N-benzyl maleimide, and N-ethyl maleimide; vinyl derivatives such asvinylcaprolactam, vinylpyrrolidone, vinylimidazole, vinylnaphthalene,and vinyl halides; vinylethers such as vinylmethyl ether; andvinylesters of carboxylic acids such as vinylacetate and vinylbutyrate.

A combination of monomers, oligomers, and/or prepolymers may also beused. The monomers, oligomers, and/or prepolymers may possess differentdegrees of functionality, and a mixture including combinations of mono-,di-, tri- and higher functionality monomers, oligomers, and/orprepolymers may be used.

For oil based inkjet inks, the dispersion medium can be any suitable oilincluding aromatic oils, paraffinic oils, extracted paraffinic oils,naphthenic oils, extracted napthenic oils, hydrotreated light or heavyoils, vegetable oils, and derivatives and mixtures thereof. Paraffinicoils can be normal paraffin types (octane and higher alkanes),isoparaffins (isooctane and higher iso-alkanes) and cycloparaffins(cyclooctane and higher cyclo-alkanes), and mixtures of paraffin oils.

Surfactants

The pigmented inkjet ink according to a preferred embodiment of thepresent invention may contain at least one surfactant. The surfactant(s)can be anionic, cationic, non-ionic, or zwitter-ionic and are usuallyadded in a total quantity less than 20 wt % based on the total weight ofthe pigmented inkjet ink and particularly in a total less than 10 wt %based on the total weight of the pigmented inkjet ink.

Suitable surfactants for the pigmented inkjet ink according to apreferred embodiment of the present invention include fatty acid salts,ester salts of a higher alcohol, alkylbenzene sulphonate salts,sulphosuccinate ester salts, and phosphate ester salts of a higheralcohol (for example, sodium dodecylbenzenesulphonate and sodiumdioctylsulphosuccinate), ethylene oxide adducts of a higher alcohol,ethylene oxide adducts of an alkylphenol, ethylene oxide adducts of apolyhydric alcohol fatty acid ester, and acetylene glycol and ethyleneoxide adducts thereof (for example, polyoxyethylene nonylphenyl ether,and SURFYNOL™ 104, 104H, 440, 465 and TG available from AIR PRODUCTS &CHEMICALS INC.).

Biocides

Suitable biocides for the pigmented inkjet ink include sodiumdehydroacetate, 2-phenoxyethanol, sodium benzoate, sodiumpyridinethion-1-oxide, ethyl p-hydroxybenzoate, and1,2-benzisothiazolin-3-one, and salts thereof.

Preferred biocides are Bronidox™ available from HENKEL and Proxel™ GXLavailable from AVECIA.

A biocide is preferably added in an amount of 0.001 wt % to 3 wt %, morepreferably 0.01 wt % to 1.00 wt %, each based on the total weight of thepigmented inkjet ink.

pH Adjusters

The pigmented inkjet ink according to a preferred embodiment of thepresent invention may contain at least one pH adjuster. Suitable pHadjusters include NaOH, KOH, NEt₃, NH₃, HCl, HNO₃, H₂SO₄, and(poly)alkanolamines such as triethanolamine and2-amino-2-methyl-1-propanol. Preferred pH adjusters are NaOH and H₂SO₄.

Humectants/Penetrants

Suitable humectants include triacetin, N-methyl-2-pyrrolidone, glycerol,urea, thiourea, ethylene urea, alkyl urea, alkyl thiourea, dialkyl ureaand dialkyl thiourea, diols, including ethanediols, propanediols,propanetriols, butanediols, pentanediols, and hexanediols; glycols,including propylene glycol, polypropylene glycol, ethylene glycol,polyethylene glycol, diethylene glycol, tetraethylene glycol, andmixtures and derivatives thereof. Preferred humectants are triethyleneglycol mono butylether, glycerol, and 1,2-hexanediol. The humectant ispreferably added to the inkjet ink formulation in an amount of 0.1 wt %to 40 wt % of the formulation, more preferably 0.1 wt % to 10 wt % ofthe formulation, and most preferably approximately 4.0 wt % to 6.0 wt %.

Preparation of a Pigmented Inkjet Ink

The pigmented inkjet ink according to a preferred embodiment of thepresent invention may be prepared by precipitating or milling thepigment in the dispersion medium in the presence of the polymericdispersant.

Mixing apparatuses may include a pressure kneader, an open kneader, aplanetary mixer, a dissolver, and a Dalton Universal Mixer. Suitablemilling and dispersion apparatuses are a ball mill, a pearl mill, acolloid mill, a high-speed disperser, double rollers, a bead mill, apaint conditioner, and triple rollers. The dispersions may also beprepared using ultrasonic energy.

Many different types of materials may be used as milling media, such asglasses, ceramics, metals, and plastics. In a preferred embodiment, thegrinding media can include particles, preferably substantially sphericalin shape, e.g., beads consisting essentially of a polymeric resin oryttrium stabilized zirconium beads.

In the process of mixing, milling, and dispersion, each process ispreferably performed with cooling to prevent build up of heat.

In the process of mixing, milling, and dispersion, each process isperformed with cooling to prevent build up of heat, and for radiationcurable inkjet inks as much as possible under light conditions in whichactinic radiation has been substantially excluded.

The inkjet ink according to a preferred embodiment of the presentinvention may contain more than one pigment, the inkjet ink may beprepared using separate dispersions for each pigment, or alternativelyseveral pigments may be mixed and co-milled in preparing the dispersion.

The dispersion process can be carried out in a continuous, batch, orsemi-batch mode.

The preferred amounts and ratios of the ingredients of the mill grindwill vary widely depending upon the specific materials and the intendedapplications. The contents of the milling mixture include the mill grindand the milling media. The mill grind includes pigment, polymericdispersant, and a liquid carrier such as water. For inkjet inks, thepigment is usually present in the mill grind at 1 wt % to 50 wt %,excluding the milling media. The weight ratio of pigment over polymericdispersant is preferably 20:1 to 1:2.

The milling time can vary widely and depends upon the pigment,mechanical means and residence conditions selected, the initial anddesired final particle size, etc. In a preferred embodiment of thepresent invention, pigment dispersions with an average particle size ofless than 100 nm may be prepared.

After milling is completed, the milling media is separated from themilled particulate product (in either a dry or liquid dispersion form)using conventional separation techniques, such as by filtration, sievingthrough a mesh screen, and the like. Often the sieve is built into themill, e.g., for a bead mill. The milled pigment concentrate ispreferably separated from the milling media by filtration.

In general, it is desirable to make the inkjet inks in the form of aconcentrated mill grind, which is subsequently diluted to theappropriate concentration for use in the inkjet printing system. Thistechnique permits preparation of a greater quantity of pigmented inkfrom the equipment. By dilution, the inkjet ink is adjusted to thedesired viscosity, surface tension, color, hue, saturation density, andprint area coverage for the particular application.

Spectral Separation Factor

The spectral separation factor SSF was found to be an excellent measureto characterize a pigmented inkjet ink, as it takes into accountproperties related to light-absorption (e.g., wavelength of maximumabsorbance λ_(max), shape of the absorption spectrum andabsorbance-value at λ_(max)) as well as properties related to thedispersion quality and stability.

A measurement of the absorbance at a higher wavelength gives anindication on the shape of the absorption spectrum. The dispersionquality can be evaluated based on the phenomenon of light scatteringinduced by solid particles in solutions. When measured in transmission,light scattering in pigment inks may be detected as an increasedabsorbance at higher wavelengths than the absorbance peak of the actualpigment. The dispersion stability can be evaluated by comparing the SSFbefore and after a heat treatment of, e.g., a week at 80° C.

The spectral separation factor SSF of the ink is calculated by using thedata of the recorded spectrum of an ink solution or a jetted image on asubstrate and comparing the maximum absorbance to the absorbance at ahigher reference wavelength λ_(ref). The spectral separation factor iscalculated as the ratio of the maximum absorbance λ_(max) over theabsorbance A_(ref) at a reference wavelength.

${SSF} = \frac{A_{\max}}{A_{ref}}$

The SSF is an excellent tool to design inkjet ink sets with a largecolor gamut. Often, inkjet ink sets are now commercialized, wherein thedifferent inks are not sufficiently matched with each other. Forexample, the combined absorption of all inks does not give a completeabsorption over the whole visible spectrum, e.g., “gaps” exist betweenthe absorption spectra of the colorants. Another problem is that one inkmight be absorbing in the range of another ink. The resulting colorgamut of these inkjet ink sets is low or mediocre.

EXAMPLES Materials

All materials used in the following examples were readily available fromstandard sources such as Aldrich Chemical Co. (Belgium) and Acros(Belgium) unless otherwise specified. The water used was deionizedwater.

SMA 1000P is a styrene maleic anhydride alternating copolymer availablefrom ATOFINA.

Raney Nickel™ is a catalysator from DEGUSSA.

WAKO V-601 is dimethyl 2,2′-azobisisobutyrate from Wako Pure ChemicalIndustries, Ltd.

MSTY or alpha methylstyrene dimer is 2,4-diphenyl-4-methyl-1-pentenefrom Goi Chemical Co., Ltd.

AA is acrylic acid from ACROS.

MAA is methacrylic acid from ACROS.

BA is butyl acrylate from ACROS.

EHA is 2-ethyl hexyl acrylate from ACROS.

STY is styrene from ACROS.

Proxel™ Ultra 5 from AVECIA.

Glycerol from ACROS.

1,2-propanediol from CALDIC CHEMIE NV.

Surfynol™ 104H from AIR PRODUCTS & CHEMICALS INC.

PY155 is the abbreviation for C.I. Pigment Yellow 155 for which Ink jetYellow 4G VP 2532 from Clariant was used.

Measurement Methods1. Measurement of SSF

The spectral separation factor SSF of the ink was calculated by usingthe data of the recorded spectrum of an ink solution and comparing themaximum absorbance to the absorbance at a reference wavelength. Thechoice of this reference wavelength is dependent on the pigment(s) used:

-   -   if the color ink has a maximum absorbance A_(max) between nm and        500 nm, then the absorbance A_(ref) must be determined at a        reference wavelength of 600 nm;    -   if the color ink has a maximum absorbance A_(max) between nm and        600 nm, then the absorbance A_(ref) must be determined at a        reference wavelength of 650 nm;    -   if the color ink has a maximum absorbance A_(max) between nm and        700 nm, then the absorbance A_(ref) must be determined at a        reference wavelength of 830 nm.

The absorbance was determined in transmission with a Shimadzu UV-2101 PCdouble beam-spectrophotometer. The ink was diluted to have a pigmentconcentration of 0.002%. In the case of a magenta ink, the ink wasdiluted to have a pigment concentration of 0.005%. A spectrophotometricmeasurement of the UV-VIS-NIR absorption spectrum of the diluted ink wasperformed in transmission-mode with a double beam-spectrophotometerusing the settings of Table 4. Quartz cells with a path length of 10 mmwere used and water was chosen as a blank.

TABLE 4 Mode Absorbance Wavelength range 240-900 nm Slit width 2.0 nmScan interval 1.0 nm Scan speed Fast (1165 nm/min) Detectorphoto-multiplier (UV-VIS)

Efficient pigmented inkjet inks exhibiting a narrow absorption spectrumand a high maximum absorbance have a value for SSF of at least 30.

2. Dispersion Stability

The dispersion stability was evaluated by comparing the SSF before andafter a heat treatment of one week at 80° C. Pigmented inkjet inksexhibiting good dispersion stability have a SSF after heat treatmentstill larger than 30.

3. Polymer Analysis

Unless otherwise specified, all polymers have been characterized withgel permeation chromatography (GPC) and nuclear magnetic resonancespectroscopy (NMR) using the following methods. Random or blockcopolymers were analyzed with NMR by dissolving them in a deuteratedsolvent. For ¹H-NMR ±20 mg polymer was dissolved in 0.8 mL CDCl₃ orDMSO-d₆ or acetonitrile-d₃ or D₂O (with or without NaOD addition).Spectra were recorded on a Varian Inova 400 MHz instrument equipped withan ID-probe. For ¹³C-NMR ±200 mg polymer was dissolved in 0.8 mL CDCl₃or DMSO-d6 or acetonitrile-d3 or D₂O (with or without NaOD addition).Spectra were recorded on a Varian Gemini2000 300 MHz equipped with aSW-probe.

Mn, Mw, and polydispersity (PD) values were determined using gelpermeation chromatography. For polymers dissolvable in organic solventsPL-mixed B columns (Polymer Laboratories Ltd) were used with THF+5%acetic acid as mobile phase using polystyrene with known molecularweights as calibration standards. These polymers were dissolved in themobile phase at a concentration of 1 mg/mL. For polymers dissolvable inwater PL Aquagel OH-60, OH-50, OH-40 and/or OH-30 (Polymer Laboratories,Ltd.) column combinations were used depending on the molecular weightregion of the polymers under investigation. As mobile phasewater/methanol mixtures adjusted to pH 9.2 with, e.g., disodiumhydrogenphosphate were used with or without the addition of neutral salts, e.g.,sodium nitrate. As calibration standards polyacrylic acids with knownmolecular weights were used. The polymers were dissolved in either wateror water made basic with ammonium hydroxide at a concentration of 1mg/mL. Refractive index detection was used.

An example is now given to illustrate the calculation of the averagecomposition of a random (=statistical) copolymer P(MAA-c-EHA).

The Mn of the copolymer was determined with GPC to be 5000.

The molar percentage of each monomer type by NMR was determined to be:45 mole % MAA and 55 mole % EHA.

Calculation:(0.45×M_(MAA))+(0.55×M_(EHA))=140.095000/140.09=total number of monomeric units in average polymer chain=36Average number of MAA units=0.45×(5000/140.09)=16 unitsAverage number of EHA units=0.55×(5000/140.09)=20 units

Thus, the average composition is P(MAA₁₆-c-EHA₂₀)

4. Particle Size

The particle size of pigment particles in pigmented inkjet ink wasdetermined by photon correlation spectroscopy at a wavelength of 633 nmwith a 4 mW HeNe laser on a diluted sample of the pigmented inkjet ink.The particle size analyzer used was a Malvern™ nano-S available fromGoffin-Meyvis.

The sample was prepared by addition of one drop of ink to a cuvetcontaining 1.5 mL water and mixed until a homogenous sample wasobtained. The measured particle size is the average value of 3consecutive measurements consisting of 6 runs of 20 seconds. For goodink jet characteristics (jetting characteristics and print quality) theaverage particle size of the dispersed particles is preferably below 150nm.

5. Calculation of % MW

The % MW is calculated as the ratio of the molecular weight of thepending chromophore group over the molecular weight of the color pigmentmultiplied by 100.

Example 1

This example illustrates that pigments in accordance with preferredembodiments of the present invention can be dispersed in inkjet inksusing a polymeric dispersant having one or more pending chromophoregroups that are smaller but similar in chemical structure to thepigment. The polymeric backbone of the dispersant was an alternatingpolymer, which is known to have poor dispersing capability.

Polymeric Dispersants DISP-1 to DISP-3

The alternating copolymer SMA 1000P was used as polymeric dispersantDISP-1 to prepare a comparative inkjet ink.

DISP-1 was then used for preparing styrene maleic acid copolymersmodified by the chromophores MC-O and MC-P.

TABLE 5 MC-O

MC-P

The synthesis is based on a kinetic study on the mono-esterification ofstyrene maleic anhydride copolymers disclosed by HU et al.,Monoesterification of Styrene-Maleic Anhydride Copolymers with Alcoholsin Ethylbenzene: Catalysis and Kinetics, Journal of Polymer Science:Part A, Polymer Chemistry, 1993, Vol. 31, pp. 691-670. The generalsynthesis scheme is represented by:

Chromophore MC-O

The synthesis of the chromophore MC-O will now be described.

Preparation of Compound MC-1C

The vessel used to carry out this reaction was a 3 necked flask equippedwith a stirrer, a cooler, and a dropping-funnel. To a solution of 13.9 g(0.1 mol) 2-nitrophenol (compound MC-1A) in 100 mL dimethylformamide wasadded 31.8 g (0.3 mol) of sodiumcarbonate. The mixture was heated to atemperature of about 150-160° C. and 16.1 g (0.2 mol) of 2-chloroethanol(compound MC-1B) was added drop-wise. After addition of the2-chloroethanol, the temperature was maintained at a temperature between150° C. and 160° C. for about 7 hours. The charge was cooled whilestirring and the formed inorganic salts were filtered off. The filtratewas concentrated by evaporation at a temperature of 40° C. until a redcolored mixture of oil and solid was formed. Then the oil was dissolvedin methylenechloride and the inorganic salts were filtered off. Thefiltrate was evaporated for a second time and the formed yellow oil waspurified by preparative column chromatography. The yield of compoundMC-1C was 79%.

Synthesis Scheme of Compound MC-1C

Preparation of Compound MC-1D

Compound MC-1D was made by catalytic reduction of compound MC-1C withhydrogen.

A reactor was filled with 18.3 g (0.1 mol) of compound MC-1C in 100 mLethanol and 1 mL of Raney Nickel™ slurry was added. The volume of themixture was set to 150 mL with ethanol and the reduction was carried outat a starting temperature of 35° C. under an initial H₂-pressure of 60bar. By shaking the reactor, the exothermic reaction started and thetemperature increased to about 60° C. After reduction, the Raney Nickel™was filtered off. The filtrate was evaporated at a temperature of 50° C.until the desired white crystalline product MC-1D appeared. The yield ofcompound MC-1D was 95%.

Synthesis Scheme of Compound MC-1D:

Preparation of Chromophore MC-O

6.9 g (45.2 mmol) of compound MC-1D was mixed with 40 mL H₂O and 10 mLmethanol. Then, 4.5 g (54 mmol) of compound MC-1F was added and themixture was stirred for 30 minutes. This is mixture A-MC-O. 9.4 g (45.2mmol) of compound MC-OA was mixed in 50 mL H₂O+50 mL methanol. 16.2 g(162 mmol) of concentrated HCl was added and the mixture was then cooledto a temperature of about 0 to 5° C. 4.05 g (58.8 mmol) of sodiumnitrite was added and the mixture was kept at a temperature between 0and 5° C. After 15 minutes the excess of nitrite was neutralized byadding 1.36 g (13.6 mmol) of sulfamic acid and a pH of 7 was obtained byadding 11.4 g (136 mmol) of sodium carbonate. The mixture A-MC-O wasadded and the mixture was stirred for 1 hour at a temperature between 0and 5° C. The yellow product was filtered and washed with methanol. Theyield of the chromophore MC-O was 44%.

Synthesis Scheme of Chromophore MC-O:

Chromophore MC-P

The synthesis of the chromophore MC-P will now be described.

Preparation of Compound MC-8B

The vessel used to carry out this reaction was a 3 necked flask equippedwith a stirrer, a cooler, and a dropping-funnel. To a solution of 140 g(1 mol) 3-nitrophenol (compound MC-8A) and 1.4 L N-methylpyrolidone wasadded 190 mL sodium methylate 30% (1.025 mol). The mixture wasdistillated at a temperature of 100° C. and 80 mbar pressure. After thedistillation, 87 mL (1.3 mol) of 2-chloroethanol (compound MC-1B) wasadded dropwise. After addition of the 2-chloroethanol, the mixture washeated to a temperature of about 120° C. for 3 hours. The reactionmixture was poured into 6 L of water with 85 mL HCl conc. The productwas filtrated. The yield of compound MC-8B was 27%.

Synthesis Scheme of Compound MC-8B:

Preparation of Compound MC-8C

Compound MC-8C was made by catalytic reduction of compound MC-8B withhydrogen.

A reactor was filled with 101 g (0.55 mol) of compound MC-8B in 700 mLethanol and 11 mL of Raney Nickel™ slurry was added. The reduction wascarried out at a starting temperature of 75° C. under an initialH₂-pressure of 46 bar. After reduction, the charge was mixed during 1hour and the Raney Nickel™ was filtered off. The filtrate was evaporatedat a temperature of 50° C. until the desired white crystalline productMC-8C appeared. The yield of compound MC-8C was 95%.

Synthesis Scheme of Compound MC-8C:

Preparation of Compound MC-P

6.9 g (45.2 mmol) of compound MC-8C was mixed with 40 mL H₂O and 10 mLmethanol. Then, 4.5 g (54 mmol) of compound MC-1F was added and themixture was stirred for 30 minutes. This is mixture A-MC-P.

6.2 g (45.2 mmol) of compound MC-PA was added to a mixture of 100 mL H₂Oand 50 mL methanol. 16.2 g (162 mmol) of concentrated HCl was added. Thesolution was cooled to a temperature of about 0 to 5° C. 4.05 g (58.8mmol) of sodium nitrite was added and the mixture was kept at atemperature between 0 and 5° C. After 15 minutes the excess of nitritewas neutralized by adding 1.36 g (13.6 mmol) of sulfamic acid and a pHof 7 was obtained by adding 11.4 g (136 mmol) of sodium carbonate. Themixture A-MC-P was added and the mixture was stirred for 1 hour at atemperature between 0 and 5° C. The stirring was continued for 3 hoursat a temperature of 20° C. The yellow product was filtered and washedwith methanol. The yield of the chromophore MC-P was 63%.

Synthesis of Polymeric Dispersant DISP-2

Polymeric dispersant DISP-2 was prepared by modifying the polymericdispersant DISP-1 with the chromophore MC-O. The resulting pendingchromophore group PC-O was linked by C* to the polymeric backbonethrough a linking group L containing an ester bond.

Preparation of DISP-2

The styrene maleic anhydride copolymer SMA 1000P was dissolved in DMA(dimethylacetamide) as a solvent. Then 5 mole % of MC-O, based on theanhydride units in the polymer, was added and the reaction was allowedto continue for 24 hours at room temperature. After 24 hours, thepolymer was precipitated with methyl t-butyl ether, isolated byfiltration and washed several times with methyl t-butyl ether. Theisolated polymer was suspended in water and the pH was adjusted to 10,using 5 N NaOH. The mixture was stirred until the polymer dissolved.Upon complete dissolution, the pH was adjusted to 2, using 6N HCl. Thepolymeric dispersant was precipitated from the medium, isolated byfiltration and dried. The degree of substitution was determined by¹H-NMR-spectroscopy and expressed as the percentage of esterified maleicacid units. The degree of substitution of DISP-2 with the chromophoreMC-O was 1%.

Synthesis of Polymeric Dispersant DISP-3

Polymeric dispersant DISP-3 was prepared by modifying the polymericdispersant DISP-1 with the chromophore MC-P. The resulting pendingchromophore group PC-P was linked by C* to the polymeric backbonethrough a linking group L containing an ester bond.

Preparation of DISP-3

The styrene maleic anhydride copolymer SMA 1000P was dissolved in DMA(dimethylacetamide) as solvent. Then 5 mole % of MC-P, based on theanhydride units in the polymer, was added and the reaction was allowedto continue for 24 hours at room temperature. After 24 hours, thepolymer was precipitated with methyl t-butyl ether, isolated byfiltration and washed several times with methyl t-butyl ether. Theisolated polymer was suspended in water and the pH was adjusted to 10,using 5 N NaOH. The mixture was stirred until the polymer dissolved.Upon complete dissolution, the pH was adjusted to 2, using 6N HCl. Thepolymeric dispersant was precipitated from the medium, isolated byfiltration and dried. The degree of substitution was determined by¹H-NMR-spectroscopy and expressed as the percentage of esterified maleicacid units. The degree of substitution of DISP-3 with the chromophoreMC-P was 1%.

Preparation of Inkjet Ink

All inkjet inks were prepared in the same manner to obtain a compositionas described in Table 6, except that different pigments and dispersantswere used.

TABLE 6 Component wt % Pigment 4.00 Dispersant 2.40 1,2-propanediol21.00 Glycerol 7.00 Proxel ™ Ultra 5 0.80 Surfynol ™ 104H 0.09 Water64.71

An ink composition was made by mixing the pigment, the dispersant andabout half of the water with a dissolver and subsequently treating thismixture with a roller mill procedure using yttrium-stabilized zirconiumoxide-beads of 0.4 mm diameter (“high wear resistant zirconia grindingmedia” from TOSOH Co.). A polyethylene flask of 60 mL was filled to halfits volume with grinding beads and 20 g of the mixture. The flask wasclosed with a lid and put on the roller mill for three days. The speedwas set at 150 rpm. After milling, the dispersion was separated from thebeads using a filter cloth. During stirring, the surfactant Surfynol™104H and the biocide Proxel™ Ultra 5, glycerol, 1,2-propanediol and theremaining water were added. This mixture was stirred for 10 minutes andfiltered. The filtration was performed in two steps. First, the inkmixture is filtered using a (plastipak) syringe with a microfiberdisposable filtercapsule with 1 μm pore diameter (GF/B microfiber fromWhatman Inc.). Then the same procedure is repeated on the filtrate.After the second filtration the ink is ready for evaluation.

Using the above method, the comparative inkjet ink COMP-1 and theinventive inkjet inks INV-1 and INV-é were prepared according to Table7.

TABLE 7 Polymeric Polymeric Chromophore Colour InkJet Ink Dispersantbackbone Group Pigment COMP-1 DISP-1 Alternating None PY155 INV-1 DISP-2Alternating PC-O PY155 INV-2 DISP-3 Alternating PC-P PY155Results and Evaluation

The spectral separation factor (SSF) was determined for each sampledirectly after preparation and was determined again after a severe heattreatment of 1 week at 80° C. The results are listed in Table 8.

TABLE 8 SSF Chromophore Color 1 week InkJet Ink Group Pigment % MW SSF80° C. COMP-1 None PY155 0 6 flocculated INV-1 PC-O PY155 55% 103 32INV-2 PC-P PY155 55% 61 37

From Table 8 it is clear that although an alternating polymer was usedas the polymeric backbone of the polymeric dispersant, the pigments weredispersed exhibiting a high dispersion quality and a good dispersionstability for the inventive pigmented inkjet inks INV-1 and INV-2.

Example 2

This example illustrates the method for preparing the polymericdispersants through uncomplicated synthesis with monomers alreadycontaining a chromophore. Since pigments exhibit low solubility in manysolvents, this complicates the synthesis of polymeric dispersants, whichis not witnessed if a chromophore is used with smaller molecular weightbut still exhibiting a resemblance with the pigment.

Chromophore MC-2

The formation of the chromophore MC-2 was accomplished by diazotation ofcompound MC-1D and subsequent coupling in the compound MC-2B.

Preparation of Chromophore MC-2

29.98 mL (0.36 mol) of concentrated hydrochloric acid was added to asuspension of 15.3 g (0.1 mol) of compound MC-1D in 300 mL water. Thismixture was cooled to a temperature of about 0-5° C. and 8.97 g (0.13mol) of sodiumnitrite was added. The diazonium-salt was kept at atemperature between 0° C. and 5° C. After 15 minutes the excess ofnitrite was neutralized by adding 3.0 g (0.03 mol) of sulfamic acid anda pH of 7 was obtained by adding 25.2 g (0.3 mol) of sodiumcarbonate.While the diazonium-salt was made, 20.7 g (0.1 mol) of MC-2B from ACROSwas dissolved in a mixture of 500 mL methanol and 10.0 mL (0.1 mol) 29%sodiumhydroxide-solution. This solution was added drop-wise into thediazonium-salt solution and a yellow suspension was immediately formed.The temperature was maintained between 0° C. and 5° C. for about 3 hoursand the yellow product MC-2 was filtered and washed with methanol. Theyield of the chromophore MC-2 was 92%.

Synthesis Scheme of the Chromophore MC-2:

Polymeric Dispersants DISP-4 to DISP-6

The polymeric dispersants DISP-4 to DISP-6 were prepared bycopolymerizing a monomer MONC already containing the chromophore groupPC-2.

Synthesis of the Monomer MONC

Ethylacetate (480 ml) was cooled to 0° C. Acrylic acid (19.0 g, 0.264mol) and 2,6-di-tert-butyl-4-methylphenol (0.2 g, 0.00088 mol) wereadded. Triethylamine (26.7 g, 0.264 mol) was added drop-wise while thetemperature was maintained between −5° C. and 0° C. Finally, benzenesulfonyl chloride (22.3 g, 0.126 mol) was added drop-wise. Triethylaminehydrochloride precipitated. The reaction mixture was allowed to stir for1 hour at 0° C. resulting in the formation of the symmetric anhydride.To this mixture N-hydroxysuccinimide (0.7 g, 0.006 mol) and MC-2 (22.3g, 0.06 mol) were added at 5° C. The reaction mixture was refluxed (78°C.) for about 17 hours. The reaction mixture was diluted with EtOAc (100ml) and extracted with distilled water (400 ml). The organic layer wasseparated and again extracted with a mixture of an aqueous solution ofhydrochloric acid and distilled water (⅕). Finally the organic layer waswashed with water and dried over MgSO₄. After evaporation of thesolvent, the residue was suspended into distilled water and stirred for45 minutes. Filtration provided a yellow solid.

Synthesis Scheme of MONC:

Synthesis of Polymeric Dispersant DISP-4

The polymeric dispersant DISP-4 was prepared by copolymerizing themonomer MONC with MAA and EHA monomers.

The synthesis was performed in a 50 ml three-necked round bottomed flaskwhich was equipped with a cooling unit, a bubble counter on top and astirring bar. 1.42 g of the monomer MAA, 3.03 g of the monomer EHA, 1.55g of the monomer MONC, 0.20 g of the initiator WAKO™ V601, 0.21 g of thetransfer agent MSTY were introduced in 23.59 g of MEK. The total weight% concentration of the monomers was 20. The reaction mixture wasdegassed by bubbling nitrogen in the solution for approximately 30 min.The flask was immersed into an oil bath and heated to 80° C. and themixture was further reacted for 20 hours. After polymerization, thereaction mixture was cooled down to room temperature. The polymer wasprecipitated in 250 ml of water followed by drying under vacuum at 40°C. for 24 hours to afford 4.5 g of yellow powder of DISP-4.(Yield=70.2%)

Analytical results of DISP-4: GPC: Mn=3893; Mw=7828; PD=2.01 (aqueousGPC; calibrated vs. PSSA-standards)

NMR: MAA/EHA/MONC molar ratio was 46/46/8. On average DISP-4 contained11 MAA monomeric units, 11 EHA monomeric units and 2 MONC monomericunits.

Synthesis of Polymeric Dispersant DISP-5

The polymeric dispersant DISP-5 was prepared by copolymerizing themonomer MONC with AA and BA monomers.

The synthesis was performed in a 50 ml three-necked round bottomed flaskwhich was equipped with a cooling unit, a bubble counter on top and astirring bar. 1.47 g of the monomer AA, 2.61 g of the monomer BA, 1.93 gof the monomer MONC, 0.20 g of the initiator WAKO™ V601, 0.21 g of thetransfer agent MSTY were introduced in 23.59 g of MEK. The total weight% concentration of the monomers was 20. The reaction mixture wasdegassed by bubbling nitrogen in the solution for approximately 30 min.The flask was immersed into an oil bath and was heated to 80° C. and themixture was further reacted for 20 hours. After polymerization, thereaction mixture was cooled down to room temperature. The polymer wasprecipitated in 250 ml of water followed by drying under vacuum at 40°C. for 24 hours to afford 3.32 g of yellow powder of DISP-5.(Yield=51.8%)

Analytical results of DISP-5: GPC: Mn=5875; Mw=10853; PD=1.85 (aqueousGPC; calibrated vs. PSSA-standards)

NMR: AA/BA/MONC molar ratio was 47/43/10. On average DISP-5 contains 21AA monomeric units, 19 BA monomeric units and 4 MONC monomeric units.

Synthesis of Polymeric Dispersant DISP-6

The polymeric dispersant DISP-6 was prepared by copolymerizing themonomer MONC with AA and EHA monomers.

The synthesis was performed in a 50 ml three-necked round bottomed flaskwhich was equipped with a cooling unit, a bubble counter on top and astirring bar. 1.23 g of the monomer AA, 3.15 g of the monomer EHA, 1.62g of the monomer MONC, 0.20 g of the initiator WAKO™ V601, 0.21 g of thetransfer agent MSTY were introduced in 23.59 g of MEK. The total weight% concentration of the monomers was 20. The reaction mixture wasdegassed by bubbling nitrogen in the solution for approximately 30 min.The flask was immersed into an oil bath and was heated to 80° C. and themixture was further reacted for 20 hours. After polymerization, thereaction mixture was cooled down to room temperature. The polymer wasprecipitated in 250 ml of water followed by drying under vacuum at 40°C. for 24 hours to afford 3.72 g of yellow powder of DISP-6.(Yield=58.03%)

Analytical results of DISP-6: GPC: Mn=4922; Mw=8320; PD=1.69 (aqueousGPC; calibrated vs. PSSA-standards)

NMR: AA/EHA/MONC molar ratio was 47/43/10. On average DISP-6 contained15 AA monomeric units, 14 EHA monomeric units and 3 MONC monomericunits.

While preferred embodiments of the present invention have been describedabove, it is to be understood that variations and modifications will beapparent to those skilled in the art without departing the scope andspirit of the present invention. The scope of the present invention,therefore, is to be determined solely by the following claims.

1. A pigment dispersion comprising: a color pigment represented byformula (I):

and a polymeric dispersant having, via a linking group covalently linkedto its polymeric backbone, at least one pending chromophore group whichhas a molecular weight smaller than 95% of the molecular weight of thecolor pigment; wherein R1 to R10 are independently selected from thegroup consisting of hydrogen, a halogen atom, a methyl group, an ethylgroup, a methoxy group, an ethoxy group, —CF₃, —COOH, —COOCH₃,—SO₂NH—C₆H₅, —CONH—C₆H₅, —CONH—C₆H₅—CONH₂, and —CONH₂; and the pigmentdispersion is an inkjet ink and the pigment comprises 0.1 wt % to 20 wt% of the inkjet ink based on a total weight of the inkjet ink.
 2. Thepigment dispersion according to claim 1, wherein the pending chromophoregroup is represented by formula (II):

wherein one of L1, L2, or L3 is the linking group and is selected fromthe group consisting of an aliphatic group, a substituted aliphaticgroup, an unsaturated aliphatic group, and a substituted unsaturatedaliphatic group; L1, L2, and/or L3, if not representing the linkinggroup, are independently selected from the group consisting of hydrogen,an alkyl group, an alkenyl group, an alkoxy group, a carboxylic acidgroup, an ester group, an acyl group, a nitro group, and a halogen; AR1and AR2 represent an aromatic group; and n represents the integer 0or
 1. 3. The pigment dispersion according to claim 1, wherein thepending chromophore group is represented by Formula (III):

wherein one of R1 to R11 is the linking group forming a covalent bondwith the polymeric backbone; R1 to R11, if not representing the linkinggroup, are independently selected from the group consisting of hydrogen,an alkyl group, an alkenyl group, an alkoxy group, an alcohol group, acarboxylic acid group, an ester group, an acyl group, a nitro group anda halogen; or R7 and R8 may together form a heterocyclic ring.
 4. Thepigment dispersion according to claim 3, wherein the heterocyclic ringformed by R7 and R8 is imidazolone or 2,3-dihydroxypyrazine.
 5. Thepigment dispersion according to claim 1, wherein the linking groupcontains at least one atom selected from the group consisting of anoxygen atom, a nitrogen atom, and a sulphur atom.
 6. The pigmentdispersion according to claim 5, wherein the linking group in unreactedform is represented by —O—CH₂—CH₂—OH.
 7. The pigment dispersionaccording to claim 6, wherein the pending chromophore group representedby formula (III) having an unreacted linking group is selected from thegroup consisting of:


8. The pigment dispersion according to claim 1, wherein the colorpigment is selected from the group consisting of C. I. Pigment Yellow155 and C. I. Pigment Yellow
 198. 9. The pigment dispersion according toclaim 1, wherein the polymeric backbone of the polymeric dispersant is ahomopolymer.
 10. The pigment dispersion according to claim 1, whereinthe pending chromophore group is present in the range of 1 to 30 percentbased on the number of monomeric units of the polymeric dispersant. 11.The pigment dispersion according to claim 1, wherein the polymericdispersant has a number average molecular weight Mn between 500 and30,000 and a polymeric dispersity PD smaller than
 2. 12. The pigmentdispersion according to claim 1, wherein the inkjet ink is a curableinkjet ink.
 13. A method for preparing the inkjet ink according to claim1, comprising the step of preparing the polymeric dispersant bycopolymerizing a monomer already containing the pending chromophoregroup.
 14. The pigment dispersion according to claim 1, wherein thepolymeric dispersant includes at least two chromophore groups.